The cost of ground source heat pumps is used by the legacy gas/Hydrogen lobby as an argument for the vast energy cost of creating hydrogen and not going all electric. Its a red herring.
Heat pumps are just fridges in reverse. They need cost no more than a fridge. So why no mass manufacture them in china and housebuilders buy them in bulk. Just look at the prices on Alibaba
There are too other costs. Pipework for underfloor heating (radiators are inefficient at the lower heat loads). Just build them into modular wooden floor plates, and the cost of digging horizontal heat collection pipes. Note you would need to dig new gas pipes for a fair comparison.
Lets not fall for the entire nation subsidizing the legacy gas industry, as ridiculous as the whale oil industry demanding a subsidy in the switchover to coal gas .
The real issue is existing housing with gas boilers. The cost benefit here is between extra insulation and the national cost of the generation of electricity to convert hydrogen. Has anyone in the legacy gas industry ever done a calculation? It may be cheaper simply to replace gas with electric boilers and offset the extra energy cost with extra capacity, as this, with extra insulation, is still more energy efficient than creating hydrogen.
Hence there is no real reason to not ban new gas boilers from 2023. This is enough time for housebuilders to build supply chains and change their standard timber framed designs.